1. Field of the Invention
This invention relates to the removal of arsenic from fluid materials, including aqueous fluids and organic fluids such as oils.
2. Description of the Art
Vast deposits of oil shale, a sedimentary marlstone, are known to exist in various areas of the world. Such deposits are found in the United States, with the more commercially important materials located in the states of Colorado, Utah and Wyoming. The geologic unit known as the Green River formation in those states contains oil shale having up to about 35 percent by weight of hydrocarbons, in the form of kerogen. Upon heating the shale ("retorting"), kerogen decomposes to produce crude shale oil vapors, which can be condensed into a synthetic crude oil and subsequently introduced into a refinery for conversion to valuable fuels, lubricants and other products.
A number of retorting processes are known, generally classified in two categories: "in situ", wherein shale is heated in chambers formed underground without removing a significant portion of the rock material, and "above ground", wherein shale is mined by conventional methods and transported to a pyrolysis device for heating. The various processes each accomplish separation of solid and liquid retort products, using techniques which are specifically designed for the particular process.
One successful above ground retorting process is shown in U.S. Pat. No. 3,361,644 to Deering, which patent is incorporated herein by reference. In this process, oil shale is fed upwardly through a vertical retort by means of a reciprocating piston. The upwardly moving oil shale continuously exchanges heat with a downwardly flowing high-specific-heat, hydrocarbonaceous recycle gas introduced into the top of the retort at about 1200.degree. F. In the upper section of the retort (the pyrolysis zone), the hot recycle gas educes hydrogen and hydrocarbonaceous vapors from the oil shale. In the lower section (the preheating zone), the oil shale is preheated to pyrolysis temperatures by exchanging heat with the mixture of recycle gas and educed hydrocarbonaceous vapors plus hydrogen. Most of the heavier hydrocarbons condense in this lower section and are collected at the bottom of the retort as a product oil. The uncondensed gas is then passed through external condensing or demisting means to obtain additional product oil. The remaining gases are then utilized as a product gas, a recycle gas as hereinbefore described and a fuel gas to heat the recycle gas to the previously specified 1200.degree. F. temperature.
In addition to shale oils, retorting processes also produce a substantially inorganic residue, generally called "spent oil shale". This material usually closely resembles the original raw oil shale in physical size and texture, but is chemically quite different. A significant chemical difference between raw oil shale and spent oil shale (except, of course, for the difference in contained organic matter) is some conversion of carbonates originally present in the oil shale to oxides. This conversion is very low in the lower temperature retorting processes, but can be complete in a high temperature process. Other transformations can occur during retorting to form certain silicate species which are not found in raw oil shales, but these silicates, being fairly inert substances, are not likely to have an effect upon the chemical reactivity of spent shale.
Some oil shale retorting processes cause the formation of a carbonaceous deposit on the surface of the shale particles, which can be combusted to recover otherwise discarded heating values. This combustion step will normally be conducted at temperatures sufficiently high to remove substantially all of the carbonate content from the spent oil shale, forming "decarbonated spent shale". Further, certain of the higher grades of oil shale contain sufficient kerogen for direct burning, omitting any need for retorting. Both decarbonated shale and the residue from direct burning of oil shale, as well as any oil shales which have been heated to a temperature above about 800.degree. F., are considered as spent oil shale for the purpose of the present invention.
In most oil shale retorting processes, arsenic components which may be present in the shale either sublime to or are pyrolyzed into vaporous arsenic-containing components. As a result, arsenic in various forms collects with the educed hydrocarbonaceous vapors and condenses with the higher molecular weight hydrocarbons in the preheated zone or, in some processes, in a condenser situated outside of the retorting vessel. When oil shale from the Green River formation is retorted, the concentration of arsenic in the produced crude shale oil is usually in the range of about 30 to 100 parts per million by weight.
Shale oil can be refined to produce valuable fuels, lubricants and the like, using many of the methods known for petroleum processing, such as catalytic cracking, hydrotreating, hydrocracking, reforming and others. Problems arise, however, due to the irreversible poisoning of expensive catalysts used in such processing, caused by the high arsenic content of the oil.
In addition to causing processing difficulties, the arsenic content limits the usefulness of shale oil even in its unrefined state, since burning an arsenic-containing fuel results in unacceptable pollution. For these reasons, it is desirable to reduce the amount of arsenic present in shale oils to the lowest possible level.
Murray et al. in U.S. Pat. No. 2,779,715, describe an arsenic-removing treatment for hydrocarbons, which requires mixing the hydrocarbon with an alkali metal or alkaline earth oxide, hydroxide, or salt which will have a pH above 7 when dissolved in water. Upon separation of the hydrocarbon, it was found to have a reduced arsenic content.
U.S. Pat. No. 2,867,577 to Urban et al. teaches a method for removing arsenic from hydrocarbons by treating with a nitrogen compound, such as ammonia, hydrazine and amines, and separating a hydrocarbon with reduced arsenic content.
Other arsenic removal methods have utilized solid absorbents, such as nickel and molybdenum components deposited on refractory oxides. Examples of such methods are disclosed in U.S. Pat. Nos. 3,804,750 to Myers et al., 3,876,533 to Myers, and 4,046,674 to Young.
Young, in U.S. Pat. No. 4,075,085, describes a method wherein a hydrocarbon feedstock is mixed with oil-soluble nickel, cobalt or copper-containing additives, heated to at least 300.degree. F., and filtered to remove arsenic. This method has been applied to crude shale oils.
Water is also recovered from the retorting process, usually as a vapor admixed with crude shale oil vapors. After retort product condensation, this water is normally separated from the oil and treated for disposal or re-use in the process. The water typically contains some arsenic, in an amount which is dependent upon the nature of the retorting process and also the form in which arsenic was present in the original oil shale.
Since arsenic is a notorious pollutant of surface and ground water systems, considerable attention has been given to its removal from industrial and mining wastes. Techniques such as precipitation (e.g., using ferric salts and lime), reverse osmosis and ion exchange have been reported as effective in arsenic removal from mine drainage. Each of these techniques, however, suffers from high costs, either in consumed reagents or in capital equipment.
In view of the high costs of the methods described and the complex nature of most of the methods, a requirement exists for a simple arsenic removal procedure which is applcable to both aqueous and organic fluids, and which does not utilize expensive reagents or equipment.
Accordingly, it is an object of the present invention to provide a simple, inexpensive arsenic removal method.
It is a further object to provide an arsenic removal method which can be used for treating both aqueous and organic fluids.
A still further object is to provide an arsenic removal method which utilizes a waste material from oil shale retorting.
These, and other objects, will appear to those skilled in the art, from consideration of the following description and claims.